In US 2004/0038459 a method is described for mixing an organic semiconductor with a polymer binder. The requirement for the polymer binder is that it has a low dielectric constant. However, as can be seen from the Table included in US 2004/0038459, the binder is present in the formulation at a weight ratio of typically less than 50%, and already in this range a significant reduction of field-effect mobility is observed compared to the pure semiconducting polymer. This document indicates that one criterion for selecting the binder is that its dielectric constant E needs to be sufficiently low to avoid lowering of the mobility as a result of increased dipolar disorder (for PS ∈=2.6). However, no mention is made of any requirements on the binder polymer to avoid the degradation in the field-effect mobility that is observed when the semiconducting component is being diluted with the binder.
The present invention is concerned with avoiding the commonly observed degradation of electrical performance upon mixing a binder polymer into the organic semiconductor formulation. We show that the key step of achieving this is to use a crystalline binder polymer.
According to a first aspect of the present invention there is provided a method for forming a semiconductor body, the method comprising: forming a mixture of an organic semiconducting material and a binder material; causing the semiconducting material to at least partially solidify; and causing the binder material to crystallize in such a way as to cause the semiconducting material to at least partially segregate from the binder material.
Preferably the step of causing the semiconducting material to at least partially solidify is performed prior to the step of causing the binder to crystallize. The step of causing the semiconducting material to at least partially solidify comprises may cause the semiconducting material to crystallize. Suitably the step of causing the binder material to crystallize comprises decreasing the temperature of the mixture.
Preferably the organic semiconducting material is a semiconducting polymer. Preferably the binder material is a semi-crystalline polymer. The organic semiconducting polymer may form a block copolymer with the polymer binder. The binder material may be semiconducting.
Preferably the mixture comprises a solvent and the method further comprises the step of removing the solvent prior to the step of causing the binder material to crystallize. Preferably the step of removing the solvent occurs prior to the step of causing the semiconducting material to solidify.
The organic semiconductor may be poly-3-hexylthiophene (P3HT) or α-quaterthiophene (4T). The binder may be isotactic polystyrene (i-PS) or high-density polyethylene (HDPE).
According to a second aspect of the present invention there is provided a method for forming a semiconductor body, the method comprising: forming a mixture of an organic semiconducting material and a binder material; causing the semiconducting material to at least partially solidify; and causing the binder material to crystallize in such a way as to cause the semiconducting material to become concentrated at the interfaces of the binder material with adjacent media.
The interfaces of the binder material may include those interfaces between adjacent layers of binder material.
According to a third aspect of the present invention there is provided a method for forming a semiconductor body, the method comprising: forming a mixture of an organic semiconducting material and a binder material; and causing the binder material to coalesce in such a way as to cause the semiconducting material to segregate from the binder material.
Preferably the step of causing the semiconducting material to solidify occurs prior to the step of causing the binder material to coalesce. Preferably the step of causing the semiconducting material to solidify comprises causing the semiconducting material to crystallize.
Suitably the step of causing the binder material to coalesce comprises decreasing the temperature of the mixture. The segregated semiconducting material and binder material may form a stratified structure.
Preferably the mixture of the organic semiconducting material and binder material is deposited onto a substrate. Preferably the step of causing the binder material to coalesce comprises causing the binder material to crystallize.
According to a fourth aspect of the present invention there is provided an electronic device comprising a semiconducting body, the body comprising: an organic semiconducting material; a crystalline binder material.
Preferably the organic semiconducting material is predominantly concentrated at the interfaces of the semiconducting body with adjacent media. The semiconducting body may be phase-segregated.
The crystalline binder material may be a dielectric, semiconductor or conductor. The organic semiconducting material may be a semi-crystalline polymer. The crystalline binder material may be a semi-crystalline polymer.
Suitably the organic semiconducting material has a melting point above that of the crystalline binder material.
The crystalline binder material may exhibit a spherrulitic microstructure.
The concentration of the organic semiconducting material in the semiconducting body may be less than 10%, 20%, 30%, 40% or 50% by weight.
Preferably the semiconducting body defines a geometric plane and the organic semiconducting material has a lamellar structure, the lamellae being oriented so as to lie generally in that plane.
The device may be a transistor, diode or photovoltaic diode.
Preferably the organic semiconducting material concentrated at the interfaces of the semiconducting body with adjacent media constitutes a continuous path for charge transport through the organic semiconducting material from a source electrode of the transistor to a drain electrode of the transistor.
According to a fifth aspect of the present invention there is provided a semiconducting layer comprising a binder and an organic semiconducting material, the organic semiconducting material being concentrated in lamellar zones that are segregated from the binder and are orientated so as to lie generally in the plane of the layer.
According to a sixth aspect of the present invention there is provided a plastic substrate comprising an organic semiconductor material and a semi-crystalline polymer binder.
The concentration of the organic semiconducting material in the substrate may be less than 10%, 20%, 30%, 40% or 50% by weight. The organic semiconducting material may be a semiconducting polymer. Preferably the substrate is flexible.
According to an aspect of the invention we disclose an electronic device comprising a semiconducting layer, wherein said semiconducting layer contains an organic semiconductor, and a crystalline binder. By using a crystalline binder the concentration of the active polymer semiconductor can be reduced to values as low as 3-5% or below without any degradation in the field-effect mobility. In the context of the present invention the term binder refers simply to a component which is mixed together with the organic semiconductor layer. Its main function is to modify the mechanical, film-forming or processing properties of the material. As such it can be a dielectric material which does not contribute directly to the charge transport in the semiconducting layer. However, it is also possible to incorporate an additional electro-optical or semiconducting function into the binder (see, for example, discussion of FIG. 16). Preferrably, the binder is a crystalline polymer. In another embodiment of the present invention, the binder is a crystalline, solution-processible small molecule or oligomeric molecule.
Preferably the organic semiconductor and the crystalline binder are segregated in the semiconducting layer.
FIG. 15a shows a schematic diagram of an electronic switching device comprising a substrate 1, on which a gate electrode 2 is deposited and patterned. A gate dielectric layer 3 is then deposited, and source-drain electrodes 4 are then patterned. Then the semiconducting layer of the device is deposited from a solution or from the melt. The semiconducting layer comprises an organic semiconductor and a crystalline binder polymer. As a result of the crystalline nature of the binder the organic semiconducting layer is driven to segregate to the interface with the substrate 5 and to the surface 7 in a vertically stratified morphology. The layer of binder 6 in between might contain a small concentration of the organic semiconductor, for example because of the some miscibility of the organic semiconductor in a molten phase of the binder. However, most of the organic semiconducting material segregates to the surface and interface of the film where it forms a continuous pathway for charge transport along the gate dielectric/semiconductor interface leading to a high mobility even for a small concentration of organic semiconductor. In this way it is possible to avoid the commonly observed degradation of electrical performance upon mixing a binder into the organic semiconductor formulation.
Preferrably, the degree of crystallinity of the binder is between 5% and 100%.
According to an aspect of the present invention we disclose a method for forming an electronic device comprising a semiconducting layer, wherein said method comprises depositing said semiconducting layer from a solution containing an organic semiconductor and a binder that is capable of crystallising. Preferably the deposition conditions are such that the binder and the semiconductor segregate in the semiconducting layer. Preferably the deposition conditions are such that the binder, or a substantial proportion of it, crystallises during and/or after deposition. Preferably, the deposition conditions are selected such that the organic semiconductor solidifies before the binder crystallizes. This preferably occurs upon removal of the solvent. The result is preferably that the semiconducting layer adopts a stratified morphology, which preferably comprises one or more sub-layers of the organic semiconductor and one or more layers of the binder. Those sub-layers may be vertically distinct: i.e. they may lie generally parallel to a substrate on which the layer may be deposited.
Without wanting to be bound by theory, the method is believed to be based on the strong enthalpic crystallization forces of the binder that expel the semiconducting component into a vertically stratified morphology to the surface and interface of the film during the crystallization of the binder.
The semiconductor and the binder may be distinct materials. Alternatively, they may be embodied as components of a single material, for example as blocks of a common molecule. The single material could comprise a polymer that includes one or more semiconducting blocks or components and one or more binder blocks or components.
According to an aspect of the present invention we disclose an electronic device comprising a semiconducting layer, wherein said semiconducting layer contains a diblock copolymer with an active semiconductor first block and a semiconducting or dielectric second block wherein the second block is a crystalline polymer.
FIG. 15a shows a schematic diagram of an electronic switching device comprising a substrate 1, on which a gate electrode 2 is deposited and patterned. A gate dielectric layer 3 is then deposited, and source-drain electrodes 4 are then patterned. Then the semiconducting layer of the device is deposited from a solution or from the melt. The semiconducting layer comprises a diblock copolymer with a semiconducting polymer block and a crystalline dielectric block. As a result of the crystalline nature of the dielectric block the semiconducting block is driven to segregate to the interface with the substrate 5 and to the surface 7 in a vertically stratified morphology. The layer 6 in between which is enriched in the dielectric block 6 might contain a small concentration of the polymer semiconductor, particularly when the film thickness is selected such that it is not possible for the semiconducting polymer to undergo microphase separation and reach the interface or surface (due to the covalent link with the dielectric which exists in this case. However, the semiconducting polymer that is able to segregate to the surface and interface of the film forms a continuous pathway for charge transport along the gate dielectric/semiconductor interface leading to a high mobility even for a small concentration of polymer semiconductor. In this way it is possible to avoid the commonly observed degradation of electrical performance for semiconducting diblock copolymers as the length of the dielectric block is increased with respect to that of the semiconducting block.
According to an aspect of the present invention we disclose a method for forming an electronic device comprising a semiconducting layer of a diblock copolymer with an active semiconductor first block and a crystalline semiconducting or dielectric second block, wherein said method comprises depositing said semiconducting layer from a solution containing the diblock copolymer. Preferably, the deposition conditions are selected such that upon removal of the solvent the first block solidifies before the second block crystallizes. Without wanting to be bound by theory the method is believed to be based on the strong enthalpic crystallization forces of the binder that expel the semiconducting component into a vertically stratified morphology to the surface and interface of the film during the crystallization of the binder.
According to an aspect of the present invention we disclose a method for forming a free-standing semiconducting film, fibre, tape, and other objects wherein said ree-standing semiconducting film, fibre, tape, and other objects comprises an organic semiconductor and a crystalline binder.
The use of crystalline-crystalline polymer blends and blockcopolymers enables new processing pathways for controlling the microstructure of functional polymers, and greater flexibility for realizing high-performance semiconducting polymer films with reduced cost, and improved mechanical and electrical properties by removing the need to design all the application requirements into the active semiconducting polymer itself. The method opens up new approaches for realizing vertically stratified structures in semiconducting polymer blends that are of interest also for a variety of other device applications such as photovoltaic cells. By diluting the relatively expensive semiconducting material with a lower cost commodity polymer without loss of performance the technique also offers significant potential for cost reduction, as materials cost in many organic-based products are dominated by the semiconducting polymer. This enables the use of free-standing semiconducting polymer substrates, fibres, tapes, and other objects that make use of the excellent mechanical properties of binder polymers such as polyethylene.
According to an aspect of the present invention there is provided a method for forming a semiconductor body, the method comprising: forming a mixture of an organic semiconducting material and a binder material; and causing the binder material to coalesce in such a way as to cause the semiconducting material to segregate from the binder material. The binder material and the conditions prevailing during the step of causing the binder material to coalesce are preferably such as to cause the binder material to adopt a phase in which it is energetically favourable for the semiconductor material to fully or substantially segregate from the binder material. The step of causing the binder material to coalesce preferably comprises causing the binder material to crystallize. The method preferably comprises the step, prior to the step of causing the binder material to coalesce, of causing the semiconducting material to solidify. The mixture may also comprise a solvent. The semiconductor body could form a semiconductor component of a semiconductor device.
In each aspect of the invention the organic semiconductor could be made up of two or more organic semiconductor materials. These could segregate into a blended phase comprising two or more such materials, or they could segregate from each other as well as from the binder. The binder could be made up of two or more binder materials. These could segregate into a blended phase comprising two or more such materials, or they could segregate from each other as well as from the semiconductor material(s).
In each embodiment the concentration of the organic semiconductor, or such of the semiconducting polymer as is active, is preferably below 20%, more preferably below 10% and most preferably below 5% or 3%, by mass or volume of the semiconducting layer or film, or of any other form of semiconducting structure that is formed.